In this specification unipolar pulses mean pulses of equal amplitude representing the sense of binary data by their polarity with the convention that the positive polarity represents the binary "1" and the negative polarity, the binary "0". By bipolar code is meant a three-level code in which the center or reference level, e.g., zero voltage, represents data of one binary sense and either outer level, whether positive or negative, represents the other binary sense with the convention that the inner level represents the binary "1" and the outer levels in alternation represent the binary "0".
In the past circuits for converting unipolar to bipolar code commonly used a center-tapped transformer in the output stage to produce positive and negative pulses from a single-polarity supply voltage. An example of this type of circuit is taught by Riffle, Clark, Chen and Gilbert in FIG. 2 (page 102, lower right corner) of their article, "A Monolithic Power PCM Repeater Circuit", published in Institute of Electrical and Electronics Engineers, Inc., Journal of Solid State Circuits, Vol. SC-14, No. 1, February 1979. Also in the same journal at page 109, Davis, Gracyk and Griffin in their article, "Design of An Integrated Circuit for the T1C Low-Power Line Repeater", teach bipolar converter technology in which a center-tapped transformer is used for unipolar to bipolar conversion. In many prior art systems the transformer is also used as a power separation filter in a simplex d-c power scheme. The use of a transformer imposes limitations on circuit design that heretofore have not been addressed. Many times the use of a transformer is incompatible with integrated circuit technology because the discrete transformer does not fit within the size constraints required by this circuit technology.
It is known that unipolar to bipolar conversion utilizes transformers to produce positive and negative pulses from a single-polarity supply voltage. It is also known that the transformers can cause size limitation problems in conjunction with integrated circuit technology. This size limitation problem is a serious one and has to be remedied if small integrated circuits are to be used.
The converter circuit of this invention eliminates the transformer from the circuit configuration. Without a transformer in the circuit, a unipolar to bipolar circuit can be constructed entirely with small integrated components.